Alkali metal hydrides find wide application in chemical industry and laboratory practice. LiH, for example, is broadly used as a strong reducing agent in chemical syntheses to prepare other hydrides, amides, isotopic compounds, and a variety of reagents for organic synthesis. LiH is also a highly desirable material for neutron shielding or moderating for mobile nuclear reactors. NaH is primarily used as a strong base in organic synthesis capable of deprotonating a range of weaker Bronsted acids to yield the corresponding sodium derivatives. It is used in the production of ethers via Williamson synthesis, alkylation of a-carbon atoms of ketones, and alkylation of amines, carboxylic acids, esters and nitriles. The carbanions produced with NaH are used in many condensation reactions which are important in the production of pharmaceutical intermediates.
Preparation of alkali-metal hydrides has been achieved and described in several earlier inventions. In one such process, hydrogen is passed over fused sodium metal at 350° C. to form NaH on the surface followed by its separation from metal with liquid ammonia. To achieve large scale synthesis of alkali metal hydrides, Freudenberg and Kloepfer invented a method in 1931 in which alkali metal is rendered into a finely divided state by several means such as by spraying fused metal through a nozzle, or by mixing the metal with a pulverulent solid diluting agents. In the procedure described in this early patent, alkali-metal is ground, for example in a ball-mill, with a selected diluent, which may consist of unreactive material such as iron powder, soda or common salts. Salts may include alkali-metal hydrides themselves to prevent cold welding and clamping, and achieve a finely divided state of the alkali metal. This process is carried out under inert conditions to prevent oxidation of active metals. This finely divided metal along with the diluent is then exposed to flowing hydrogen at elevated temperatures between 200-300° C. to form corresponding alkali metal hydrides. Some aspects of this procedure, however, may not be conducive to a large scale operation. For instance, transfer of highly active milled metal from one reactor to another, or heating the powders under hydrogen at elevated temperatures may constitute significant safety hazards.
Subsequent work by V. L. Hansley (U.S. Pat. Nos. 2,372,670; 2,372,671; 3,222,288) to produce alkali metal hydrides was aimed mainly at improving the rate of reactions between the metals and hydrogen by introducing small quantities (not exceeding 10 wt %) of so called “activators” such as aromatic hydrocarbons, other petroleum based hydrocarbons or fatty acids having more than 8 carbon atoms. Addition of such organic activators/diluents would however require further processing and may compromise the purity of the product. In a similar process, alkali-metal hydrides are also prepared as their dispersions in a variety of inert liquids such as hydrocarbons, ethers and tertiary amines used as a reaction medium. In this process, organic compounds of elements of groups 4 and 14 such as butyl titanate or triethyl silicol are used as dispersants. Once again, such processes result in products that require further processing to obtain pure hydride products. Currently, alkali metal hydrides are industrially produced by heating pure metal above their respective melting temperatures under hydrogen atmosphere. For example, LiH is produced from the reaction of lithium metal and hydrogen gas at more than 500° C.
More recent work by J. C. Snyder (U.S. Pat. Nos. 3,387,948; 3,387,949; 3,485,585), describes a process of preparation of alkali-metal hydrides that involves reaction of alkali-metal with hydrogen in the presence of a transition-metal catalyst in the form of free-metal or its hydride or halide salts. These reaction components are introduced into a mill-like reactor and suspended in an inert organic liquid that may or may not act as a solvent for the product. The reaction mixture is then heated at temperatures between 80-200° C. under hydrogen pressure of 70-350 bar with constant comminution of the reaction mixture. It is understood that comminution via milling is employed in this method mainly to assist in mass transfer, and the reaction is mostly thermal in nature and not promoted by the mechanical energy as generated in the high-energy ball-milling (HEBM) process.
A mechanochemical synthesis of alkali-metal hydrides (AH where A=Na, K, Cs and Rb) by HEBM under hydrogen pressure carried out without addition of a catalyst or a dispersing agent is described in the work by Elansari et al [JALCOM 2001, 329, L5]. Although the authors report the initiation and significant progress of reaction in their case, after 12 hours of milling, it was necessary to place the reactors under elevated temperatures (about 120° C.) for the reaction to complete.
U.S. Pat. Nos. 2,372,670; 2,372,671; 3,222,288; 3,387,948; 3,387,949; and 3,485,585 describe two-step synthesis of simple alkali metal hydrides using ball milling to provide metal (e.g. Na pieces) pieces of suitable size followed by reduction of the metal pieces at elevated temperatures. U.S. Pat. No. 1,796,265 describes a one-step and two-step process to make simple alkali metal hydrides using ball milling in flowing hydrogen at 180 to 300 degrees C. for making NaH.